1. Field of the Invention
The present invention relates to a continuous process for the production of base compositions for addition-crosslinking silicone compositions which have improved stability in relation to viscosity increase, SiH degradation, change of reactivity, and also additive degradation after storage.
2. Background Art
Those skilled in the art of silicone elastomers are aware that silicone compositions comprising filler, for example liquid silicone rubbers (LSR) are composed mainly of base compositions. For the production of liquid silicone rubbers, base compositions are mixed homogeneously with a vulcanizing agent, and also with other typical constituents of addition-crosslinking silicone compositions, which serve to establish certain final properties of the silicone elastomers obtained after the vulcanization process.
One prior-art method for the production of base compositions is based on use of prehydrophobized fillers, homogeneously mixed and kneaded together with appropriate organopolysiloxanes. Another method for producing base compositions uses hydrophilic fillers, which are hydrophobized in-situ during what is known as the compounding procedure, with use of suitable treatment agents. In both instances, the incorporation procedure usually takes place in a planetary-geared dissolver or a kneader, an example being a sigma kneader or an open divided-trough kneader, these being unsuitable in principle for a continuous production process. The batch processes described, and in particular here the in-situ process, feature a disadvantageous space-time yield, high energy consumption, and the need to use inert gas because of the risk of formation of explosive mixtures, and they moreover have very high capital investment costs and labor costs. Kneaders operating batchwise can produce silicone compositions with high storage stability from base compositions comprised of organopolysiloxanes and of prehydrophobized oxidic reinforcing fillers, and the silicone elastomers produced using these base compositions feature a very balanced mechanical property profile.
There are numerous known prior-art technologies for producing silicone compositions by way of continuous process steps. The continuous steps in the process can be related either to the continuous production of base compositions or else to the continuous production of finished silicone compositions, the latter being composed in most cases of one or more components.
Silicone compositions which are composed of base compositions produced continuously, and which use prehydrophobized fillers, generally have lower stability when compared with silicone compositions comprising base compositions produced batchwise.
Comparatively low residence time of the organopolysiloxane and of the filler in continuous processes for the production of base compositions often leads to incomplete breakdown of agglomerates of the filler, and the result of this can be inhomogeneity in the silicone elastomer, or poor transparency, as previously stated in EP 1 203 650 B1.
The reduced stability of silicone compositions produced by prior art methods from continuously produced base compositions can become apparent, for example, as an increase in the viscosity of the silicone compositions after storage, and this occurs particularly at elevated temperatures, for example during transport of the silicone compositions.
A comparatively low residence time of the organopolysiloxanes and filler in continuous processes for the production of base compositions can also lead to inadequate deactivation of the surface of the filler, a possible result of this being undesired reactions in the finished silicone composition. By way of example, if organohydropolysiloxanes are used as crosslinking agents in the finished silicone compositions, an increased level of degradation of SiH groups can be observed, with evolution of hydrogen. This is attended by a considerable risk of explosion when oxygen is present. An associated change in the architecture of the network also creates the risk of altering the property profile of the silicone elastomers obtained after the vulcanization process. In self-adhesive silicone compositions, another possible result of an inadequately deactivated surface of the filler is undesired reactions of reactive groups at the surface of the filler with additives such as adhesion promoters, inevitably leading to impairment of adhesion properties.
EP 0 462 032 A1 describes a continuous in-situ process for the production of base compositions which are suitable for the production of silicone compositions crosslinkable by way of a polyaddition reaction. The compounding process takes place in a twin-screw extruder via continuous and simultaneous feed of an organopolysiloxane bearing vinyl groups, of a filler, and also of a liquid polysilazane, and water.
EP 0 776 751 A1 describes a continuous in-situ process for the production of base compositions by producing the base composition in a machine using a biaxial system of continuous extrusion and mixing. The machine using a biaxial system of continuous extrusion and mixing has a double-lumen inlet tube by way of which the hydrophobizing agent, composed of hydrolyzable organosilicon compound and water, and the filler are fed by way of the outer lumen, and the organopolysiloxane is added by way of the inner lumen.
U.S. Pat. No. 6,749,786 B2 describes a continuous in-situ process for the production of base compositions for liquid silicone rubbers (LSR) with low volatiles content, by using an extruder with an L/D ratio (length to diameter) which is at least greater than 70. The devolatilization process takes place at a plurality of apertures positioned shortly prior to the extruder outlet.
All of the in-situ processes described have the disadvantage of high emissions (problem of exhaust gas) which occur on all kneading machines and are difficult to control. Another factor with continuous in-situ processes is that there are only limited opportunities for targeted control of the hydrophobization process, and a relatively high level of product quality variation can therefore be observed. Another disadvantage of in-situ processes derives from the ever-present risk that explosive mixtures will be formed.
EP 0 807 509 A1 discloses the continuous production of storage-stable addition-crosslinking liquid silicone rubbers, using a specific cascade of kneading machinery. The devolatilization of the base compositions produced by means of the kneading machine takes place by way of a simple devolatilizing container, but this does not encompass any aftertreatment of the base composition by way of a specifically designed aftertreatment apparatus with introduction of shear force and with high average residence time. Although the silicone compositions produced from the base compositions feature improved stability with respect to viscosity increase on storage, this is still not entirely satisfactory since it is still possible to observe an undesired viscosity increase, leading to reduction in the shelf life of the finished silicone compositions. When liquid silicone rubbers are processed by the injection-molding process, increased viscosity can lead to problems attributed to the change in rheological properties. By way of example, another problem known to those skilled in the art can also arise at metering apparatuses if inadequate storage stability has led to excessive viscosity. There is moreover also a considerable need for improvement in the stability of the silicone compositions produced from the base compositions, with respect to SiH degradation and degradation of additives, for example adhesion promoters or organically based stabilizers. A further disadvantage of the process described in EP 0 807 509 A1 is that the maximum volume flow rates that can be achieved by that continuous process are 300 kg/h. Marked impairment of the stability of the base compositions is observed at any higher throughput.
In summary, it can be stated that none of the continuous processes known hitherto for the production of base compositions for silicone compositions satisfactorily complies with the requirements placed upon these silicone compositions, in particular liquid silicone rubbers processed by injection molding.